1. Field of the Invention
The non-destructive technique to pole BaTiO.sub.3 crystal calls for a slight stress to force the domains to re-orient, which then eliminates the 90.degree. domains in the crystal. By adjusting the voltage and frequency this method may can be applied to the lump crystals of BaTiO.sub.3.
In the solidification process of BaTiO.sub.3, the temperature continues to fall. When the temperature drops to Curie Temperature (about 130.degree. C.), the crystal structure will change from cubic to tetragonal, and the corresponding point group from 3 mm to 4 mm. In the crystal's changing process, the barium and titanium atoms displace position opposite to the oxygen atoms in the [001] direction. The movement will force the titanium atom, originally situated at the center of TiO.sub.6.sup.-8 octagon, to deviate towards a certain oxygen atom's position, resulting in the positive and negative atoms' gravity center to fall on different points in producing dipole moment. At the same time, the direction movement also creates a spontaneous polarization, endowing the crystal with ferroelectric effect. However, since the polarization may be oriented in any of the six pseudodoubic [001] directions of the cubic structure, in the cooling process of crystal, crystals at the different positions will have varied polarization directions, leading to the domains of the entire crystal under room temperature to be non-single domain. It forms the 90.degree. and 180.degree. domains 1, 2 as shown in FIG. 1. In the 90.degree. domains 2, the c-axis of each domain is vertical with each other; therefore the reflection indices in the same direction are inconsistent, resulting in scattering when light passes the crystal. Consequently, the 90.degree. domains 2 can be easily seen as shown in FIG. 2. Nonetheless, for 180.degree. domains, the c-axis in each domain is antiparallel with each other. Thus, unless an etching method is used to reveal different degrees of etching for the positive and the negative domains, it is not discernible by sight.
2. Description of the Prior Art
As the 90.degree. domains 2 in the crystal scatter the light and the 180.degree. domains offset the electro-optic effect, the efficacy of optical experiments is considerably reduced. Consequently, before conducting optical test, it is imperative to turn these multi-domains into a single one. The customary method for eliminating 180.degree. domains 1 is to apply an electric field to the c-face of a BaTio.sub.3 with temperature slightly below the curie point. In terms of the 90.degree. domains 2, three methods are available presently for the elimination. One of which is uniaxial pressure. The method administers pressure to the a-axis of the crystal, forcing the domain walls to move towards the crystal surface. The crystal surface can be polished to remove the multiple domains on the outer layer, as the 90.degree. domains 2 generally cannot be completely eliminated by one pressure. The step has to be repeated until all 90.degree. domains are removed, which wears off the crystal.
Moreover, the pressure administered must be great. In 1992, NaKao, et al. (Jpn. J. Appl. Phys. Vol. 31, Pt. 1-9B, pp. 3117), the pressure reported was as high as 2.times.10.sup.7 Newton/m.sup.2. Thus, once a crystal contains any internal flaw, the crystal is likely to fracture, especially for crystals with a diameter of less than 1 mm. A little force will break the crystal into little pieces. This makes the uniaxial method inappropriate for the current materials.
Furthermore, in 1991, Garrett, et. al. (Ferroelectrics, Vol. 120, pp. 167) had also used immersed etching to remove the 90.degree. domains 2. The concept of the method arises from using acidic solution to remove the strain layer of the crystal surface created by machine cutting and polishing (The layer is likely to impede domain movement). This enables electrical poling to easily move the atom position, and along with electric field direction, turn it into a single domain. The advantage of the method is that it does not require great stress, and eliminates the occurrence of a flaw in the crystal. However, during the corrosion process, the crystal surface is worn off and turned coarse, making it not suitable for a crystal fiber.
In 1992, F. Ito (Appl. Phys. Lett. Vol. 6131-18, pp. 2144) proposed another method to eliminate the 90.degree. domains. The method is quite simple. It calls for placing crystal on the electrodes, and subjecting it to high voltage for tens of hours. The greatest advantage of the method is its capacity to turn both 90.degree. and 180.degree. domains to a single domain. The drawback is that the 90.degree. domain walls are difficult to move, and therefore difficult to eradicate completely. From the above description, it is evident that the elimination of 90.degree. domains has been a problematic issue for the field.